Have Waterco Micron Sand Filter: After cleaning out both filter baskets which were quite full,the pressure in my system dropped to around 20 -30 kpa after running at around 50 kpa since install around 2 yr ago..... Completed a backwash (assuming there was dirt in filter medium) then noticed pressure dropped to zero and a strange clinking noise a bit like marbles coming from filter....(Ran the rinse cycle for around a minute as per instructions in between backwash and before returning to normal)....Any Ideas ???